A current mirror is a circuit designed to emulate a current through one active device by controlling the current in another active device of a circuit, keeping the output current constant regardless of loading. The current being emulated can be a varying signal current. Current mirrors can be used in analog circuits as biasing, reference, and simple current-operator circuit elements. Common usages for current mirrors are in output amplifiers and linear regulators. Line regulation is the capability to maintain a constant output voltage level on an output channel despite changes to the input voltage level.
Current mirrors can be characterized by a transfer ratio, in the case of a current amplifier, or the output current magnitude, in the case of a constant current source. Current mirrors can also be characterized by an alternating current (AC) output resistance and by a minimum voltage drop, referred to as dropout voltage. The output resistance determines how much the output current varies with the output voltage applied to the current mirror. The dropout voltage is the smallest possible difference between the input voltage and output voltage to remain within the intended operating range of the device. This minimum voltage is dictated by the need to keep the output transistor of the current mirror in active mode.
Two important characteristics of output amplifiers and linear regulators are minimum supply voltage at maximum rated output current and output voltage variation from minimum to maximum supply voltage. There is usually a tradeoff between these two characteristics; minimum gate length of common source output metal-oxide-semiconductor (MOS) device minimizes dropout voltage while increasing supply sensitivity at low output current. Supply sensitivity is increased at low output current due to reduced amplifier open loop gain and lower effective “Early” voltage of output device operating in moderate or weak inversion. Increasing open loop amplifier or regulator direct current (DC) gain reduces DC supply sensitivity, but stable feedback requires gain to “roll-off” as frequency increases. This results in increased supply sensitivity with increasing frequency.